In the packaging manufacturing industry, packaging is made from a sheet of card generally in several steps. This is why known processing machines of the prior art are traditionally made up of several successive workstations through which each sheet is moved sequentially. In practice, each sheet is conveyed individually from one workstation to another by pulling it via its front edge, after leaving the rest of the sheet not held in any particular way.
In order for the sheet nonetheless to maintain a certain degree of flatness as it decelerates on arriving at a workstation, it is known practice to brake its rear portion during the sheet introduction phase by using a suction bed. Installed transversely in close proximity to the entry to the workstation, such a braking device performs its function by restraining the rear portion of the sheet using suction, while at the same time allowing it progressively to slide as its front portion is driven forward.
There are also processing machines in which the sheets are moved over relatively long distances, for example to pass through a station in which no particular operation is performed. A notable example here might be printing machines using hot foil stamping, commonly known as foil stamping machines, wherein the sheets are transferred from the platen press to the delivery station, via the foil feed and recovery station. Whatever the case, in such a situation, it is important not only to brake each sheet as it decelerates, but also to hold it flat throughout the whole duration of the transfer.
In order to address this difficulty, one idea has been placing a vacuum chamber under the sheet transfer pathway to act as a holding device, i.e. both to support and to brake the sheet. Such a vacuum chamber can be likened to some extent to a large-size suction bed, in which the suction orifices are concentrated chiefly in the downstream part, whereas the rest of the chamber acts essentially as a support. This chamber is usually mounted with an ability to move in longitudinal translation so that it can be moved to suit the format of the sheets being conveyed, in the knowledge that its optimum position of use is as close as possible to the location at which the sheet will ultimately become immobilized at the end of transfer. It should be noted that, in the case of the foil stamping machine cited as an example, this location corresponds precisely to the location at which the stack forms in the delivery station.
This type of holding device does, however, have the disadvantage of being active only in line with the chamber, such that when the latter is to be moved into the most downstream portion of the sheet transfer pathway, it will then no longer be able to perform the holding function in the most upstream portion. In addition, the monolithic structure of the suction chamber naturally has a tendency to limit the effectiveness of such a holding device in the upstream part of the transfer pathway. This is because as a sheet approaches the upper surface of the chamber, an air cushion is created between the two of them and that significantly disrupts the fall and then the firm holding-down of said sheet.